This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Warfarin is a widely used drug for a number of clinical conditions requiring anticoagulant such as atrial fibrillation. However warfarin has substantial interindividual variability in dose requirements and a narrow margin between efficacy and serious toxicity. The goal of this study is to determine prospectively whether the presence of DNA Variants affect the action of anticoagulants (specifically Wafarin). This will shed light on the value of pre-prescription genotyping in improving control. This is a sub-study of the Pharmacogenics of Arrhythmia Therapy Ion Channel Variants and QT-Prolonging Antiarrhythmics-Anticoagulant Study performed in conjunction with Vanderbilt University. The sub-study involves the use of Anticoagulants (warfarin) only and not the QT prolong nation agents.